The invention relates to a clock modulator for modulating a basic clock signal to form a system clock signal, the system clock signal driving microprocessors and/or other digital circuits and, at the same time, generating an interference spectrum with interference spikes.
In clock generators of microprocessors and/or other digital circuits, use is made of quartz crystals in oscillators for generating a stable clock frequency. A circuit of this type generates interference characterized by very narrow spikes. The interference energy is thus concentrated at a few frequencies, these spikes reaching high levels. This is undesirable, however, for reasons of electromagnetic compatibility (EMC). It is known from the prior art to distribute the interference energy between a very large number of spectral lines with a correspondingly lower level by using clock modulators which alter the clock instants in a predetermined and defined manner. In this case, an oscillator generates a basic clock signal which is stable in terms of frequency and phase and is altered in a clock modulator to form a system clock signal in such a way that the edges are no longer separated by a constant time interval. The Implementation of known clock modulators in digital circuits is restricted to dimensioning the clock modulator in the design phase. Later it can, at best, be switched on and off. However, the interference spectrum of the isolated system clock signal may differ from the interference spectrum caused by the system clock signal in the digital system which it drives. It is disadvantageous here that it is not possible to predict the interference spectrum caused by the system clock signal in the entire digital system and whether the maximum tolerable interference values are not exceeded by this interference spectrum.
The object of the invention, therefore, is to specify a clock modulator with which the maximum interference values permitted are not exceeded by a system clock signal in a digital system.
This object is achieved according to the invention by virtue of the fact that the modulator can be adapted as a function of the interference spectrum of the system clock signal in the digital system in such a way that the amplitudes of the interference spikes of the interference spectrum are reduced and thus do not exceed the maximum tolerable interference values. As a result of this, it is advantageous that it is possible to reduce the influence of circuitry resonances, which may be caused e.g. by the individual components of the circuit, the circuit layout and/or the surroundings of the circuit. Furthermore, the modulator can be adapted in the event of circuit changes such as e.g. the use of other components or layout changes.
By altering the switching pulse spacings, in particular by altering the possible position of the individual switching pulses, it is possible to shift the spikes of the interference spectrum to frequencies where they have less of an interfering effect. Thus, for example, the interference spikes may be shifted to frequencies at which zeros were previously situated, and the level of the interference spikes can thus be reduced.
By virtue of a random number generator which can generate a plurality of random number sequences, that setting which does not exceed the permissible interference levels can be found in a simple manner simply by changing over and testing the different random sequences.
Delay elements connected in series, taps for tapping the system clock signal being arranged upstream and downstream of the delay elements, make it possible to alter the spacings of the respectively neighboring switching pulses by a changeover to a different tap. The spacing of the possible position of the individual switching pulses of the system clock signal can be varied in a simple manner by adjusting the delay times of the delay elements.
By switching in or out a number of delay elements as a function of the number generated by the random number generator, a reproducible interference spectrum is generated, in particular with cyclically recurring numbers.